A backlight unit is used to illuminate the target such as an LCD display panel. An LCD display device employs either one of two types of backlight configurations as a backlight unit; the direct type and the edge light type (light guide plate type).
With the direct type, fluorescent tubes, i.e., a light source, are arranged directly below the liquid crystal panel to be illuminated. This allows fluorescent tubes to be increased with the change in the display screen size, thus achieving a sufficient brightness. In this case, however, the backlight unit is prone to an uneven brightness between areas having a fluorescent lamp and others not. Moreover, the direct type backlight unit must be built with sufficient strength. For example, the backlight case is fabricated with a metal plate. Then, a reflective sheet is affixed to the inner surface of the backlight, with a plurality of straight tube lamps arranged thereabove.
With the edge light type, on the other hand, a fluorescent lamp is arranged at the edge of a light guiding body made, for example, of a clear acrylic plate. This type of backlight unit takes advantage of multireflection in the light guiding body to use one surface thereof as an area light source. The edge light type has a reflector at the back of the straight tube lamp and L-shaped lamp. Although the display device using the edge light type backlight unit can be reduced in thickness, the light guiding body of the large-size model becomes excessively heavy. Besides, upsizing of the device makes it difficult to secure sufficient screen brightness.
The aforementioned features are the reasons why, in general, the direct type backlight unit is used for a large-screen liquid crystal display device, whereas the edge light type backlight unit is used for those with a small screen.
The fluorescent lamps used for the backlight unit as described above are driven by a high voltage of 1 KV at a high frequency of 50 to 70 KHz to achieve even and high brightness. At this time, the fluorescent lamps develop uneven brightness, i.e., uneven brightness, in the form of a brightness gradient between the high- and low-voltage sides as a result of a leak current. This problem is caused by the following reason. The fluorescent lamps are driven by a high voltage at a high frequency. This causes the air layer to act as a stray capacitance and leads to a leak current flowing from the fluorescent lamps to the lamp reflector and the surrounding metal objects. As a result, the current flowing into the low-voltage side of the fluorescent lamps diminishes. This causes the low-voltage side to illuminate relatively less brighter than the high-voltage side.
Therefore, if the fluorescent lamps are long, the leak current rises proportionally to the length thereof. In the presence of a large leak current, the farther the fluorescent lamps are from the drive circuit, the darker they become. This constitutes the cause of uneven brightness. That is, the larger the liquid crystal display device, the more likely the difference in brightness occurs between the high- and low-voltage sides of the lamps. It can be said that the technique allowing the realization of a backlight unit with minimal uneven brightness is essential.
FIG. 18 is an explanatory view of the brightness characteristic of fluorescent lamps, illustrating an example of the brightness distribution in the longitudinal direction (i.e., in the direction of voltage application) of the fluorescent lamps generally used for a backlight type liquid crystal display device.
As shown in FIG. 18, the fluorescent lamps have a brightness gradient whose relative brightness diminishes from a high-voltage side H to a low-voltage side L. The brightness drop is particularly noticeable near the edge of the low-voltage side L. The brightness distribution curve itself also varies depending on the shape of the fluorescent lamps, the length of the fluorescent tube, the drive voltage or the drive frequency. Basically, however, the fluorescent lamps develop uneven brightness in the form of relatively low brightness at the low-voltage side L as compared with the high-voltage side H.
FIG. 19 is a graph showing the brightness distribution characteristic in the longitudinal direction (in the direction of voltage application) of the fluorescent lamps having the brightness gradient shown in FIG. 18 when the drive voltage is further raised. In the example of FIG. 19, the brightness of the fluorescent lamps at the center and low-voltage side L is roughly equal. However, the brightness is relatively higher near the edge at the high voltage side H. For example, assuming that the brightness is 100 at the center and the low-voltage side, the brightness is relatively higher or 115 to 125 at the high-voltage side H. The brightness, highest at the edge of the high-voltage side H, gradually declines toward the center of the fluorescent lamps.
The display screen also develops uneven brightness due to uneven brightness developed by the fluorescent lamps in the longitudinal direction as described above. As a technique to reduce such uneven brightness in the display screen, the liquid crystal display device using a backlight is known as shown below.
FIGS. 20A and 20B are explanatory views of an example of the liquid crystal display device having a conventional direct type backlight unit. FIG. 20A illustrates a side cross-sectional schematic configuration of the LCD device, whereas FIG. 20B illustrates a plan schematic configuration of the fluorescent lamps, i.e., the light source of the backlight unit.
As shown in FIGS. 20A and 20B, the backlight unit has a plurality of fluorescent lamps 101, reflectors 102 adapted to reflect the light from the fluorescent lamps 101 and an optical diffusion unit 103 provided at the front of the fluorescent lamps 101 and adapted to diffuse the light directly incident from the fluorescent lamps 101 or that reflected by the reflectors 102. The backlight unit is used to illuminate a liquid crystal panel 104 provided at the front (surface side) thereof through the optical diffusion unit 103.
With the aforementioned backlight unit, the fluorescent lamps 101 are arranged in sets of two such that the high-voltage side of one lamp is adjacent to the low-voltage side of the other to compensate for uneven brightness in the lamps 101 and achieve a display screen with even brightness.
That is, as shown in FIGS. 20A and 20B, the backlight unit is provided with a plurality of sets (S1, S2, S3 and beyond) of the two fluorescent lamps 101, with the high-voltage side H of one lamp adjacent to the low-voltage side L of the other lamp. Such a configuration cancels out uneven brightness resulting from each fluorescent lamp, thus eliminating uneven brightness on the display screen and achieving an even display.
A liquid crystal display device in Patent Document 1 is disclosed as an example with the high- and low-voltage sides H and L arranged adjacent to each other.
Further, the technique as shown in FIGS. 21A and 21B is available that is associated with the liquid crystal display device operable to improve the reflectance of the light from the backlight. FIGS. 21A and 21B illustrate another example of the backlight unit in a conventional liquid crystal display device. FIG. 21A illustrates a side cross-sectional schematic configuration of the backlight unit, whereas FIG. 21B illustrates a plan schematic configuration of the inside of the unit, with the optical diffusion sheet, provided on the backlight unit surface, removed. In FIGS. 21A and 21B, reference numeral 201 denotes linear fluorescent lamps, 202 optical diffusion sheets, 203 a reflection sheet and 204 a reflection layer and 205 an enclosure.
The backlight unit shown in FIGS. 21A and 21B has the reflection layer 204, made of a high reflectance material such as aluminum, on the inner surface at the bottom of the enclosure 205 further at the back of the reflection sheet 203 provided at the back of the linear fluorescent lamps 201 to efficiently enhance the brightness. Here, of the light incident on the reflection sheet 203, the fraction that passes through the sheet 203 without being reflected is reflected again by the reflection layer 204 back toward the reflection sheet 203, rather than disappears or becomes diffused at the back of the reflection sheet 203. This ensures efficient use of the light passing through the sheet 203 from the back, thus enhancing the brightness.
In general, a foamed PET (Poly Ethylene Terephthalate) sheet is often used for the direct type reflection unit (equivalent to the reflection sheet 203 described above). The foamed PET reflection sheet is manufactured by foaming PET to produce fine air bubbles within the sheet. The light incident on the foamed PET sheet is refracted by the air bubbles to regress and emerge again from the incident side. Such a light reflection takes advantage of the refraction characteristic between the PET material and the air in the air bubbles, thus minimizing light loss and achieving a high reflectance reflection unit, despite the use of an inexpensive member. In addition to the above, other materials may be alternatively used including those coated on the surface with a high reflectance material such as silver or aluminum.
For example, while the reflection sheet 203, formed with a foamed PET sheet as described above, achieves a high reflectance, part of the incident light from the light source passes through the foamed PET sheet to the rear side (back side opposite to the light source). This leads to reduced light utilization efficiency. To improve these points for enhanced light utilization efficiency, the reflection layer 204 made of a high reflectance material such as aluminum is provided on the inner surface of the enclosure 205 at the back of the reflection sheet 203 to reflect the light passing through the reflection sheet 203 with the reflection layer 204. Part of the reflected light from the reflection layer 204 passes again through the reflection sheet 203 and emerges on the front side (light source side). This ensures improved light utilization efficiency.
An edge light type backlight device using a light guide plate is disclosed, for example, in Patent Document 2 as the backlight device having another reflection layer stacked at the back of the reflection sheet as described above.
Further, Patent Document 3 discloses a technique that changes the leak current flowing between the high- and low-voltage sides of the fluorescent tube in an edge light type backlight unit to suppress the uneven brightness of the screen. With this backlight unit, the fluorescent lamp is shaped to have straight tube portions in one piece; the one portion running along one of the longer sides of the light guide plate and the other portions each running along one of the shorter sides of the plate. The reflector, provided on the straight tube portion at the high-voltage side of the fluorescent lamp, i.e., one of the tube portions running along the shorter sides of the light guide plate, is formed with a white reflecting member, whereas the reflector at the low-voltage side is deposited on the inside with silver. Such a configuration changes the leak current flowing between the high- and low-voltage sides, thus securing a proper fluorescent lamp length to generate necessary brightness over the rectangular screen and minimizing the difference in brightness between the left and right sides of the screen.
Further, the problem here derives from the driving at a high frequency. Therefore, the method is under consideration to drive the fluorescent lamp at the lowest possible frequency for increased the impedance of the stray capacitance and reduced leak current, thus eliminating uneven brightness.
Description will be given next of the problems associated with the conventional techniques described above.
With the liquid crystal display device described in Patent Document 1, the fluorescent lamps are arranged parallel with each other in sets of two such that the high-voltage side H of one lamp is adjacent to the low-voltage side L of the other. At this time, because of the proximity between the high-voltage side terminal of one fluorescent lamp and the low-voltage side terminal of the other lamp adjacent thereto, discharge may occur between the two electrodes. This renders the stable discharge of the fluorescent lamps itself extremely difficult and possibly deteriorates the reliability of the device.
Moreover, the high- and low-voltage terminals of the fluorescent lamps are disposed separately on both sides of the display screen. This requires two inverter power circuits, resulting in higher cost. Further, the thinner and larger the display device, the more difficult it is to make wiring connections to the fluorescent lamps. As a result, additional measures are required to ensure wiring safety and prevent the current leak.
With the backlight device of Patent Document 2, on the other hand, if the brightness distribution of the fluorescent lamp is not uniform in the longitudinal direction, the entire display screen may develop uneven brightness as a result of the uneven brightness of the fluorescent lamp. This makes it difficult to control the brightness distribution. In particular, the GND side (low-voltage side) is prone to current leak from the fluorescent lamp. This results in high brightness only at the high-voltage side of the fluorescent lamp and low brightness at the GND side.
In the case of Patent Document 3, provision of the white reflector only on one of the shorter sides of the fluorescent lamp alone cannot compensate for the brightness gradient inherently present in the fluorescent lamps. The fluorescent lamp invariably develops a brightness gradient at least along its longer sides. This results in uneven brightness in the liquid crystal display device. If the fluorescent lamp is longer as a result of the upsizing of the liquid crystal display device, the aforementioned problem becomes more noticeable.
Further, while the method of lighting the lamps at a lower drive frequency could be possible to the extent that thermal runaway does not occur in the transformer, an excessively low frequency design could degrade the reliability. Besides, lowering the drive frequency will result in larger components such as the transformer.
In light of the foregoing, the present invention was conceived and the object thereof is to provide a backlight unit operable to compensate for the brightness difference between the high- and low-voltage sides of the fluorescent lamps, provided as a light source, and to ensure an even brightness of the outgoing light, and a liquid crystal display device operable to ensure an even brightness over the entire display screen.
Patent Document 1: Japanese Laid-Open Patent Publication No. H11-295731
Patent Document 2: Japanese Laid-Open Patent Publication No. H08-335048
Patent Document 3: Japanese Laid-Open Patent Publication No. H10-112213